1. Field of the Invention
The present invention relates to a bone fixation system and method for fracture fixation of bone.
Conventional bone fixation systems include a bone fracture fixation plate for placement on a bone so as to bridge a fracture therein and a set of screws inserted through the plate into the bone. The system works by drawing the fracture fragments to the plate, and if the plate is designed with “compression” holes, the fracture fragments can be made to compress against each other to promote primary bone healing. However, the angular relationship between the plate and screws is not fixed and can change postoperatively. As such, this can lead to misalignment and poor clinical results.
2. Prior Art
One method of securing the screws to a bone plate involves the use of so-called “locking screws.” A locking screw has a male thread on an outer surface of its head that interfaces With a female thread on the plate to lock the screw to the plate. Bone plates having threaded holes for accommodating locking screws are known. For example, German Patent Application No. 43 43 117 discloses a bone plate with threaded holes for locking screws. As the relationship between the locking screws and the plate is fixed, locking screws provide a high resistance to shear or torsion forces. However, locking screws have a limited capability to Compress bone fragments.
Another approach to construction of a bone plate involves use of “combination holes.” Combination holes in the bone plates have a domain for non-locking screws and another domain for locking screws. Here, the locking screws can only be applied in a direction perpendicular to the plate; (see, e.g., U.S. Pat. No. 6,469,701 and U.S. Pat. No. 7,354,441). However, only a one locking or a non-locking screw can be applied in each of these “combination” holes along the bone plate.
Another bone plate hole configuration involves a “figure-eight” hole; see, e.g., Universal Locking System available from Zimmer Holdings, Inc. (Warsaw, Ind.) and 1 Bone and Joint Surgery, 89(7) 2007. Here, a figure-eight-type hole in the bone plate has two parallel threaded domains. A locking screw can be mated to one domain of the hole or to the other domain of the same hole. In each case, the locking screw can be applied only perpendicular to the bone plate. Further, only one screw can he received in each of these figure-eight-type holes along the plate.
In yet another approach, the bone plate has individual locking holes for mating individual locking screws; see, e.g., MIS Technique available from Zimmer Holdings, Inc. (Warsaw, Ind.) Here, the individual holes are oriented alternating in one direction and in another direction (in the plane transverse to the longitudinal axis of the plate) away from the perpendicular to the plate. However, for such a configuration, half of the screw holes may not be suitable for use. In the worse case scenario, none of the holes can be used.
A more recent development in bone fracture fixation disclosed in U.S. 2009/0228010 A1, published Sep. 10, 2009, utilizes a bone fixation system including a bone plate having one or more bi-directional combination holes each of which can accommodate two bone screws, one in each domain of the same hole, one of which is a so-called by-pass screw. hi this type of system, the head of each by-pass screw is cut along a chord to provide a flat extending parallel to the screw shaft which provides clearance for the head of the other screw received in the other domain of the same hole.
While this last system has certain advantages, it also has a drawback in that the by-pass screw must he screwed into one domain of a combination hole in the bone plate so that the flat side of its head faces the other domain of that hole to provide the needed clearance for the second screw being installed in that other domain. In many instances, in order to do this, the by-pass screw may have to be undertightened or overtightened into the underlying bone in order to bring the head fiat in register with the other domain of the same hole, If that screw is undertightened, the bone plate may not be drawn sufficiently to the bone to stabilize the bone fracture. On the other hand, if that screw is overtightened, the screw threads may he stripped from the bone.
Another disadvantage of using bone screws with by-pass heads is that it increases the size of the screw inventory required to he maintained in hospitals, clinics and the like. The use of such screws with by-pass heads also complicates the surgical implantation of the plates in that the surgeon has to be be sure that he is installing the correct screw in each screw hole of the bone plate.